Electrophotographic recording elements with half-tone screen coatings thereon

ABSTRACT

AN ELECTROPHOTOGRAPHIC RECORDING ELEMENT COMPRISES A PHOTOCONDUCTIVE LAYER ON A RELATIVELY CONDUCTIVE SUBSTRATE, AND A COATING OF A PARTIALLY LIGHT-TRANSMITTING MATERIAL ON THE PHOTOCONDUCTIVE LAYER. THE COATING OF THE LIGHT-TRANSMITTING MATERIAL IS IN A PATTERN OF A HALF-TONE SCREEN, COVERING ONLY A PORTION OF THE PHOTOCONDUCTIVE LAYER, SO THAT THE RECORDING ELEMENT PROVIDES HALF-TONE TYPE-PRINTS IN AN ELECTROPHOTOGRAPHIC PROCESS.

DBC. 14, 1971 P. J. DONALD 3,627,526

ELECTROPHOTOGRAPHIC RECORDING ELEMENTS WITH HALF-TONE SCREEN COATINGSTHEREON Filed DBC. 29, 1959 INVIL'N'IUK FHJLJPJUNAL United States PatentO 3,627,526 ELECTROPHOTOGRAPHIC RECORDING ELE- MENTS WITH HALF-TONESCREEN COAT- INGS THEREON Philip `Ioseph Donald, Woodbury, NJ., assignorto RCA Corporation Filed Dec. 29, 1969, Ser. No. 888,248 Int. Cl. G03g5/04 U.S. Cl. 96-1.5 1 Claim ABSTRACT OF THE DISCLOSURE BACKGROUND OFTHE INVENTION This invention relates generally to electrophotographicrecording elements, and more particularly to electrophotographicrecording elements with half-tone screen coatings thereon. The novelelectrophotographic recording elements are particularly useful forcopying continuous-tone (continuously variable shades of gray) images byelectrophotographic methods.

Most electrophotographic recording elements have relatively short grayscales and by themselves reproduce continuous-tone images poorly. It hasbeen proposed to reproduce a continuous-tone image on anelectrophotographic recording element by employing an external halftonescreen during the exposure operation in an electrophotographic process.While such a procedure is satisfactory for some applications, it leavessomething to be desired in others. For example, in projecting ahalf-tone screened light image, the individually projected elements ofthe half-tone screen should be clearly resolvable. Hence, the opticalsystem employed for projecting the image imposes a limitation on `boththe resolution and the number of resolvable elements to be copied. Ifdirect screening is used, whereby the half-tone screen is placed incontact with a charged electrophotographic element, there is a danger ofdisplacing some of the charge of the recording element in the process,thus spoiling the resulting reproduction.

Each of the novel electrophotographic recording elements provideshalf-tone type copies of continuous-tone originals and eliminates theneed for an external halftone screen, thereby obviating theaforementioned disadvantages in the use of external half-tone screens.

SUMMARY OF THE INVENTION The novel electrophotographic recording elementcomprises a relatively conductive substrate, a photoconductiye layerthereon, and a coating of a partially light-transmltting material in thepattern of a half-tone screen on a portion of the photoconductive layer.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a fragmentary plan view ofone embodiment of the novel electrophotographic element;

FIG. 2 is a fragmentary cross-sectional View of the recording elementillustrated in FIG. 1, taken along the line 2-2 and viewed in thedirection indicated by the arrows;

FIG. 3 is a fragmentary plan View of another embodiment of the novelelectrophotographic recording element;

FIG. 4 is a fragmentary cross-sectional view of the recording elementillustrated in FIG. 3, taken along the line 4-4 and viewed in thedirection indicated by the arrows; and

FIG. 5 is a fragmentary cross-sectional View of the novel recordingelement illustrated in FIG. 3, taken along the line 5 5 and viewed inthe direction indicated by the arrows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing,there is shown an electrophotographic recording element 10 of the typecommonly employed in the electrophotographic art. The recording element10 comprises a relatively electrically conductive substrate 12, such aspaper or metal foil, and a relatively electrically insulating (indarkness) photoconductive layer 14 adhered to the substrate 12. Thephotoconductive layer 14 comprises a photoconductor, such as zinc oxidein a suitable resin binder. The photoconductive layer 14 may also be dyesensitized to render the photoconductive layer 14 responsive to light ina desired spectral range.

Recording elements of the type thus far described have a relativelyshort gray scale in comparison to photographic films and do not, bythemselves, reproduce continuous-tone images well in a typicalelectrophotographic printing system. For example, if the recordingelement thus far described were uniformly charged with an electrostaticcharge and exposed through a photographic, stepwedge gradient, lightfilter (a transparency of successive steps of light transmission rangingfrom black, through various shades of gray, to clear) only a few stepswould appear on the developed recording element. It is well recogniZed,that a recording element should have the characteristic of a long grayscale to reproduce continuoustone images faithfully.

Means are provided to increase the gray scale of the recording element10. To this end, the recording element 10 is provided with a coating ofa partially light-transmitting material on portions of thephotoconductive layer 14 in the pattern of a half-tone screen. In FIGS.1 and 2, the coating of partially light-transmitting material, in thepattern of one kind of half-tone screen, is shown in a pattern of dots16. In addition to being partially light-transmitting, the dots 16possess the characteristic of changing the time of an electrostaticdischarge across both it and the underlying photoconductive layer 14, inresponse to light of a given intensity directed onto the recordingelement, in comparison to the electrostatic discharge across thephotoconductive layer 14 alone. Preferably, the change in the time ofelectrostatic discharge across both a dot 16 and the underlyingphotoconductive layer 14, in response to light of a given intensity,should be longer than the time of electrostatic discharge across thephotoconductive layer alone.

Examples of materials that are suitable for coating the surface of thephotoconductive layer 14 of the recording element 10 in patterns ofhalf-tone screens include milled lithographie inks, as follows:

'Example I (white ink) Anatase titanium dioxide, dry 61 lb. 0 oz..Alumina hydrate, dry 5 lb. 12 OZ. Long oil alkyd resin, 100% 32 lb. 8oz. Cobalt linoleate drier, 3% metal 0 lb. l2 oz.

100 lb. 0 oz.

Example II (medium yellow ink) #l Transparent varnish 20 lb. 0 oz. #2Transparent varnish 4 lb. l2 oz. Transparent Varnish 2 lb. 0 oz. #3Transparent varnish 56 lb. 8 oz. Gloss white, dry 13 lb. 8 oz. Aluminahydrate, dry l lb. 12 oz.

On last pass over mill add:

#7 Lithographie varnish 1 lb. 8 oz.

100 lb. 0 oz.

Example III (reddish blue ink) #l Lithographie varnish 29 lb. 8 oz. #0Lithographie varnish 5 lb. 0 oz. #3 Lithographie varnish 1 0 lb. 8 oz.Phosphotungstic lake of methyl violet, dry 16 lb. 8 oz. Bronze blue, dry25 lb. 8 oz. Alumina hydrate, dry 6 lb. 0 oz. Paste drier 7 lb. 8 oz.

100 lb. 0 oz.

tExarnple lV (white ink) Percent by wt. Titanium dioxide 70 Linseedvarnish 30 Cobalt drier Trace `Coatings of these materials in the formof half-tone patterns can be applied to the photoconduct-ive layer 14 ofthe recording element 10 by lithographic printing techniques. Thus, theaforementioned inks can be printed on the photoeonductive layer 14 froma lithographie plate. The coatings can also be applied to thephotoconductive layer 14 by silk screen techniques, well known 1n theart. The thickness of the dried half-tone coating may vary in the rangebetween 0,01 mil and 1 mil, but preferably should be in the order ofabout 0.1 mil or less. The thickness of the coating should be such that1t is partially light-transmitting,

While it is desirable that the color of the half-tone coating besubstantially similar to that of the photoconduetive layer 14, as forexample the white ink of Example I when the photoconduetive layer 14 iswhite, it has been found that colored lithographie inks in partiallylighttransrnitting coatings, such as described in Examples II and II-I,can also be used to produce acceptable halftone images. Theaforementioned and other suitable inks are described in the book,Printing and Litho Inks, by Herbert Jay Wolfe, th edition, published byMacNair- Dorland Co., New York, NY`

Regardless of the particular pattern of the half-tone screen coating, itshould be a regular or random pattern substantially equivalent to ahalf-tone screen of between 50 and 250 lines per inch, and the percentof the area of the photoconductive layer 14 covered by the coating maybe between and 90%, depending on the images to be copied. The coating ofdots 16, for example, may comprise 125 dots per linear inch, regularlyor randomly spaced, may cover 50% of the area of the photoconductivelayer 14, and may be of any color if the overall appearance of therecording element is not considered objectionable. The particularhalf-tone screen coating to be used in any particular case may bedetermined 4 by the criteria considered for the selection of a contactscreen in a photographic process.

The novel recording element 10 is adapted for use in a typicalelectrophotographic process. For example, the surface of the recordingelement 10, including the photoconductive layer 14 and the coat-ing ofdots 16 thereon, is uniformly electrostatically charged negatively inthe dark by any electrostatic charging means well known in the art. Theuniformly charged surface is exposed to a light image to selectivelydischarge it and to provide thereon an electrostatic latent image. Thelatent image is then developed by any suitable electroscopie toner, wellknown in the art, to provide a visible image of the light image.

In theory, it is believed that since each dot 16 is only partiallylight-transmitting, only a portion of the light impinging thereonaffects the underlying portion of the photoconductive layer 14 on whichthe dot 16 is coated. Therefore, the portion of the photoconductivelayer 14 directly underneath a dot 16 is not rendered as conductive inresponse to a given intensity of light (impinging on the overlying dot16) as is a portion of the photoconductive layer 14 not covered by a dot16. Hence, each dot 16 decreases the response of the photoconductivelayer 14 beneath it to light and extends the over-all response of therecording element 10 to a greater range of light intensity from theoriginal image than would be possible without the half-tone pattern ofdots 16.

With a coating of partially light-transmitting material in the patternof a half-tone screen on a recording element, as described herein, it ispossible to extend the gray scale of the photoconductive layer 14, andthereby produce a half-tone reproduction with better resolution of graytones than possible without the use of the coating.

Referring now to FIGS. 3, 4, and 5, there is shown another embodiment ofa novel electrophotographic recording element 20 wherein parts similarto those in the recording element 10 are designated by the samereference numerals. The recording element 20 comprises a substrate 12and a photoconductive layer 14 of the type previously described. Acoating of light-transmitting material on the photoconduetive layer 14comprises a halftone screen in the pattern of two sets of parallelstrips intersecting each other at an angle` Thus, for example, one setof strips 22 is disposed at right angles to the other set of strips 24.Each of the strips 22 and 24 is between 0.01 mil and 1 mil in thickness(in a direction normal to the surface of the photoconductive layer 14)and can comprise a light-transmitting material such as described suprain Examples I, II, III, and IV. The strips 22 and 24 provide a patternof a half-tone screen that may be substantially equivalent to between 50and 250 lines per inch, and may cover an area of between 10% and 90% ofthe photoconductive layer 14` In coating the strips on the surface ofthe photoconductive layer 14, one of the sets of strips, such as the setof strips 24, is coated first and the other set of strips 22 is coatedover the strips 24. Thus, the thickness of the cross-over portions,hereinafter called points 26, Where the strips 22 and 24 cross eachother is relatively greater than the thickness of any one of the strips(measured in a direction normal to the major surface of thephotoconductive layer 14). With this arrangement, the cross-over points26 of the strips 22 and 24 are less light-transmitting than the otherportions of the strips 22 and 24 and exclude more light from an image totheir underlying portions of the photoconductive layer 14 than do thesingle strips 22 and 24 alone. For example, let it be assumed that thesurface of the recording element 20 is uniformly charged negatively withan electrostatic charge, -Let it also be assumed that a given intensityof light just completely discharges the unscreened (noncoated)photoconductive layer 14. Since the strips 22 and 24 are only partiallylight-transmitting, the same quantity of light that just discharges theunscreened portions of the photoconductive layer 14 does not dischargethe underlying portions of the photoconductive layer 14 beneath thecoated strips 22 and 24. Hence, the coating on these latter portionsstill retain a portion of the electrostatic charge and can attractelectroscopic developer.

Again, let it be assumed that a given intensity of light just completelydischarges the. electrostatic charge on a single thickness of either ofthe strips 22 or 24. This latter quantity of light does not dischargethe electrostatic charge at the cross-over points 26 because thelight-transmitting characteristic of the cross-over points 26 is lessthan that of a single strip 22 or 24. Hence, the coating of thecross-over points 26 retains an electrostatic charge when theelectrostatic charge on the other portions of the coating of the strips22 and 24 is just completely discharged. Thus, the half-tone screencoated recording element 20 is responsive to a greater range of lightintensities than a non-coated recording element, thereby effectivelyextending the gray scale of the coated recording element for providingdeveloped prints of improved continuous-tone in an electrophotographicprocess.

What is claimed is:

1. In an electrophotographic recording element of the type comprising asubstrate and a photoconductive layer of a substantially uniformthickness thereon, said recording element being adapted to be rstelectrostatically charged with a uniform charge and then to beselectively discharged by exposure to a light image, the improvementcomprising:

optical lter means permanently adhered to selected areas of said layer,opposite said substrate, rendering said selected areas less sensitive tolight impinging on said recording element, said selected areas being inthe pattern of a half-tone screen,

said optical lter means comprising a coating of a partiallylight-transmitting ink having a thickness of between 0.01 and 1.0 miland a characteristic such that the time of electrostatic discharge ofsaid um` form charge through both said coating and said layer,

in response to light of a given intensity, is different 40 in comparisonto the time of discharge of said uniform charge through said layer only,in response to said light of given intensity,

said coating comprising a tirst plurality of substantially parallelstrips disposed in one direction, and a second plurality ofsubstantially parallel strips disposed at an angle of substantially 90to said one direction and crossing said rst plurality of strips in aregular pattern substantially equivalent to a half-toned screen ofbetween and 250 lines per inch and covering between 10% and 90% of onemajor surface of said layer, and

the combined thickness of said irst and said second plurality of stripsat their cross-over points being greater than the thickness of any oneof said strips alone, whereby said cross-over points are less lighttransmitting than any one of said strips alone, so that saidphotoconductive layer of substantially uniform thickness may be exposed,through said regular pattern, by light of three different intensities,in response to light of a given intensity impinging on said recordingelement.

References Cited UNITED STATES PATENTS CHARLES E. VAN HORN, PrimaryExaminer U.S. Cl. X.R.

